The invention relates to a method of manufacturing a cathode ray tube having a cone part and a screen part, the method comprising the steps of aligning the cone part with respect to the screen part, sealing the cone and screen parts together, said sealing process comprising at least a cooling step.
A frit jig assembly comprising alignment means for supporting a cone part of a cathode ray tube is known from U.S. Pat. No. 3,329,422, in which a sealing frit jig is provided to minimize or avoid relative movement of the parts during the sealing process. In known manufacturing methods of sealing the display screen to the one part of the cathode ray tube, the cone part is positioned in the sealing frit jig. Next an enamel paste is applied to the edge of the cone part, and the display screen is placed onto the cone. Hereafter a heating step of about 440.degree. C. occurs to form a proper seal between the display screen and the cone part.
It is important that the display screen and the cone part are correctly positioned with respect to each other. To achieve this the sealing frit jig is provided with alignment pins. These alignment pins are applied at three points at the outer circumference of the cone part as well as at the circumference of the display screen.
The alignment pins comprise metal and therefore have a coefficient of thermal expansion different from the glass from which the screen and the cone parts have been made. The disadvantage of this method is that during the cooling phase of the device the glass tube scratches along the alignment pins and consequently becomes damaged. This damage may lead to weak spots in the tube and during the evacuation process of the tube, the tube might implode. It has been proposed to use a soft material for the pins, for example graphite, which does not damage the tube. However, such a material has the drawback that it easily wears off and consequently a frequent adjustment and position correction of the alignment pins is necessary. Another drawback of graphite is that it might introduce unwanted contaminating particles into the display tube.